1. Field of the Invention
The present invention relates to a servo circuit for a magnetic disk apparatus which performs speed control over a servo object based on an error between a target speed and a real speed. More particularly, the present invention relates to a servo circuit which performs speed control by passing a seek current, corresponding to the error between the target speed and the real speed, to a voice coil motor and to a speed signal generating circuit for finding the real speed.
2. Description of the Related Art
Servo circuits are used in magnetic disk apparatuses for positioning magnetic heads on specific tracks. In such servo circuits, it is desired to reduce the time required for the speed control and to shorten the access time and to obtain stable positioning even with higher speeds.
In particular, in recent years, higher speeds have been achieved during the seek access operations, which move and stop a head in a disk apparatus at a target track position. Along with the higher speeds of see access operations, much higher speeds have been sought for so-called "one difference" seek access operations wherein the heads are moved to the adjoining track position. A demand has therefore arisen for a servo circuit with a high response commensurate with the higher speeds of "one difference" seek operations.
Explaining the related art in more detail, in a servo circuit, speed control is used up to near the target position, then position control is switched to so as to control the positioning to the target position. However, the frequency characteristic of the servo object has a resonance point, i.e., a resonance point of the servo arm such as the carriage of the servo object. If the frequency of a speed control signal, explained later, appears in the band which covers this resonance point, mechanical oscillation appears during the seek operation and therefore there is no hope for improvement of the floating stability of the magnetic head and the stability after switching to fine control.
The frequency band of an amplifier of a speed error detection circuit, explained later, is determined by the time constant of a resistor and capacitor of a filter. The cut-off frequency must be made smaller than the resonance point. In recent years, however, higher speed access has become desirable. With a "one difference" seek operation, the acceleration/deceleration current cycle of the speed control signal becomes close to the cut-off frequency. Therefore, the current waveform, in particular, the current waveform during acceleration, becomes corrupted and no further current can be passed. Since it is difficult to raise the resonance point, the problem arises that it is difficult to shorten the time for speed control.
In other related art, provision is made of a servo loop for speed control which generates a target speed in accordance with a difference of a current track position from a target track and supplies to a voice coil motor a seek current corresponding to the difference between the target speed and the real speed. Here, the target speed has the characteristic of maintaining a predetermined maximum speed level when the difference is above a predetermined amount and of falling in proportion to a mean value of the difference, for example, when the difference is under a predetermined amount. The target speed becomes the minimum speed level in the case of a difference of one track.
However, with such a servo circuit, since acceleration is performed by passing a seek current corresponding to the difference between a target speed and real speed, determined by the difference at the start of the seek operation, when the target speed level is extremely small, such as with a "one difference" seek operation, the rise of the acceleration current flowing to the voice coil motor becomes slow.
That is, in a "one difference" seek operation, when the seek current begins to flow at the start of the seek operation and the real speed appears, the target speed level is extremely small. Thus, the difference between the real speed and target speed disappears and the seek current is suppressed. As a result, the seek current at the time of acceleration becomes triangular in form, the rise of the acceleration current deteriorates, and the access time of a "one difference" seek operation becomes longer.
In still another servo circuit, provision is made of an additional speed error preparing circuit. This circuit, however, sometimes suffers from loop resonance, which prevents a normal seek operation. Such loop resonance occurs more easily the larger the seek difference. However, if this is prevented by limiting the bands of component circuits so as to try to suppress the oscillation component, the time for the speed control becomes longer and a high speed seek operation, in particular, a high speed seek operation for a small amount of difference such as a difference of one track, becomes difficult.